The invention relates to an ink-jet recording sheet, particularly relates to paper for ink-jet printing capable of recording a character or a color image each having a high image quality on both of the sides thereof.
Recently, the quality of image printed by an ink-jet printer is becomes to be equal to that of the image obtained by the silver halide photography accompanied with a rapid progress of the ink-jet recording technology.
Various kinds of color prints can be made by the ink-jet printer-accompanied with the spreading of ink-jet printers and digital cameras, and the use of the print printed by the ink-jet printer is diversified recently.
One of such the uses of the print is ink-jet recorded images formed on the both sides of recording sheet. The recording sheet capable of recording images on the both sides thereof is suitable to reduce the number of paper sheet for resource saving and to prepare a new color print to be made in a booklet form or a various types of cards such as a post card.
Some recording sheet having ink-jet recording layers on the both sides thereof have been proposed.
Japanese Patent Publication Open to Public Inspection (hereinafter referred to JP O.P.I.) No. 56-148584 discloses a method for preventing the stain on the back surface by using recording sheet having layer containing porous inorganic pigment particles on the both sides thereof to inhibit the penetration of the ink to the opposite side of the paper.
JP O.P.I. No. 2-270588 discloses ink-jet recording sheet excellent in the feeling of paper quality and prevented in the penetration of ink to the back side, in which ink absorption layers are provided on the both sides of the paper and the ink absorption layer is made thin for preventing the peeling off of the ink absorption layer.
JP O.P.I. No. 8-174996 discloses that ink-jet recording sheet can be obtained by providing an ink absorption layer mainly composed of an inorganic pigment and a binder on one side of the base paper and immersion or coating a cationic substance to the other side of the paper. On this paper, a clear image can be recorded on one side and an ink-jet record and a writing by various kinds of writing materials can be made on the other side, and the penetration of the ink to the opposite side of the paper is prevented. Such the recording sheet is mainly used as a card or a post card.
JP O.P.I. No. 9-286116 describes ink-jet recording sheet having ink absorption layer on the both sides of a substrate different from each other in the dot diameter formed when an ink droplet is adhered thereon. Ink-jet recording sheet without curling or ink penetration to the back surface can be obtained by such the methods.
Ink-jet recording sheet capable of forming high quality images on the both sides thereof has been investigated by the inventors. As a result of that, it has been found that the image formed on one side of the paper is transparently looked on the back side through the paper, or paper through image visibility, cannot always be sufficiently inhibited even when the dye fixing ability of the ink absorption layer is increased.
The paper through image visibility tends to be occurred when a black image, a character or picture, is printed on one side and no or low density image is printed on the other side of the paper.
The paper through image visibility cannot be sufficiently inhibited even when the image is recorded by a pigment ink and colored pigment particles are substantially fixed on the surface of the ink absorption layer and almost particles of the pigment are not penetrated into the ink absorption layer.
The paper through image visibility can be improved some degree by raising the opacity of the ink absorption layer by the addition of a coarse pigment particle or the use of an inorganic pigment having a high refractive index. In such the case, however, the density of the image printed by the ink-jet is considerably lowered and the paper through image visibility cannot be controlled only by the increasing of the dye fixing ability and the opacity of the ink absorption layer.
It has been found by the inventors that the paper through image visibility can be reduced without loss of the image density by improving both of the support and the ink absorption layer.
The object of the invention is to provide an ink-jet recording sheet in which the lowering of image density is small and the image is difficultly seen through the paper when images are printed on the other side of the paper by ink-jet printing.
The invention and the embodiments thereof are described below.
Ink-jet recording sheet comprising a support having an opacity of not less than 94% according to JIS P 8138 and ink absorption layers each containing a cationic substance and provided on the both sides of the support.
Ink-jet recording sheet comprising a support having a Kobb Test permeability of not more than 10 g per square meter determined according to JIS P 8140 using pure water containing 30% by weight of diethylene glycol for a contacting time of 10 seconds and a opacity of not less than 94% according to JIS P 8138, and ink absorption layers each containing a cationic substance and provided on the both sides of the support.
The support is preferably a hydrophobic support.
The support is preferably a support comprised of a paper substrate covered with polyolefin resin layers on the both sides thereof, and at least one of the olefin resin layers contains a white pigment.
Each of the ink absorption layers provided on both sides of the support preferably includes at least one porous layer.
The ratio of D1/D2 is preferably within the range of from 0.90 to 1.1 wherein D1 and D2 are each the diameter of dot formed by a droplet of ink adhered on the ink absorption layer provided on one side of the support and that of the dot formed on ink absorption layer provided on the other side of the support, respectively.
Ink-jet recording sheet comprising a plastic film support containing a white pigment and having a opacity according to JIS P 8138 of not less than 94% and ink absorption layers containing a cationic substance each provided on the both sides of the support.
Ink-jet recording sheet comprising a support comprised of paper support having a weight of from 100 g to 250 g per square meter covered with a polyolefin resin containing a white pigment, which has an opacity according to JIS P 8138 of not less than 94%, and ink absorption layers each containing a cationic substance and provided on the both sides of the support.
The cationic substance is preferably a compound having a quaternary ammonium salt group.
The support of the ink-jet recording sheet of the invention has an opacity according to JIS P 8138 of not less than 94%.
When the opacity is less than 94%, the image tends to be seen through the paper when the recording sheet is observed from the opposite side. The opacity of the support is preferably not less than 95%, particularly not less than 96%.
Paper, white plastic film, clothes, and paper laminate with plastic resin on one or both sides thereof are usable as the support having an opacity of not less than 94%.
The opacity of paper support of not less than 94% can be attained by increasing the weight of the paper or addition of white a pigment into the paper. The weight of paper is usually from 100 g to 300 g per square meter even though the weight may changed depending on the presence of the white pigment. The weight is usually from 150 g to 300 g, preferably from 200 g to 300 g, more preferably from 220 g to 300 g, when no white pigment is used.
When the white pigment is used, the weight is usually from 100 g to 250 g, preferably from 150 g to 200 g, even though the weight is considerably changed depending on the amount of the white pigment.
Titanium oxide, barium sulfate, zinc oxide, silica and calcium carbonate are usable as the white pigment to be used in the paper. Examples of preferable hydrophobic support include a plastic film and a paper support laminated with plastic resin on one or both sides thereof.
When the plastic film support is used, a white plastic film is preferred in which the white pigment is added into the resin of film to make the opacity to 94% or more.
As the white pigment, titanium oxide and barium sulfate are usable. In such the case, there are two ways to from the white plastic film, in one of them the opacity is made by the white pigment it self and in another of them the opacity is made according to the scattering by foams formed in the course of production of the film using the white pigment as the nuclear of the foams.
For the plastic film, a polyolefin resin such as polyethylene and polypropylene, a polyester resin such as poly(ethylene terephthalate) and poly(ethylene naphthalate), poly(vinyl chloride) and cellulose triacetate are preferably used. The thickness of the plastic film is usually from 120 to 300 xcexcm, preferably from 150 to 250 xcexcm.
The using amount of the white pigment is usually from 1 to 50%, preferably from 5 to 30%, by weight of the plastic resin, even though the amount may be changed depending on the diameter or kind of the white pigment.
The plastic film may be one constituted by laminating two or more plastic film. In such the case, the kind if the films to be laminated may be the same or different from each other.
Furthermore, a paper support covered with plastic resin on one or both sides thereof may also be used. In such the case, one or both sides of a paper support having a weight of from 100 to 250 g, preferably from 120 to 220 g, is covered with the plastic resin to make the opacity of the support to not less than 94%.
The foregoing plastic resin to be used in the plastic film support can be used. Polyolefin resin is preferably used since the polyolefin resin is easily extruded in a molten state on the paper support to produce the resin laminated paper support. Preferable examples of the polyolefin resin include polyethylene, polypropylene, polyisobutylene, and a copolymer mainly composed of ethylene or propylene, and polyethylene resin is most preferable. The thickness of the layer of the preferable resin laminated on the paper support is usually from 5 to 60 xcexcm, preferably from 10 to 50 xcexcm. In this case, the thickness of the layer on one of the surface and that on the other surface may be the same or different.
The support may be one produced by coating a dispersion of hydrophobic resin on the paper support to form a hydrophobic resin layer. The dispersion of hydrophobic resin can. be produced by emulsion polymerization of one or more kinds of copolymerizable monomers such as methyl acrylate, ethyl acrylate, propyl acrylate, butyl acrylate, methyl methacrylate, ethyl methacrylate, propyl methacrylate, butyl methacrylate, acrylamide, styrene, hydroxyethyl methacrylate, vinyl chloride, vinyl ether, ethylene, divinylbenzene, vinyl acetate and butadiene.
In this case, the thickness of the plastic resin layer is usually from 2 to 40 xcexcm, preferably from 3 to 20 xcexcm.
It is preferable to contain the forgoing white pigment in one of the paper and the plastic resin to make the opacity to not less than 94%. It is particularly preferable to contain the white pigment in the plastic resin.
According to the invention, the paper through image visibility is sufficiently inhibited just after the recording by the ink-jet printing. However, in the case of a support sufficiently absorbing humid such as a paper support is used, the dye is penetrated into the support and the paper through image visibility tends to be raised when the print is stored under a condition of high humidity and temperature. It is found that a recording sheet using a support having a relatively low humid absorbability is preferably to improve such the problem.
Accordingly, it is preferable that the use of a support having a humid absorbability of not more than a specified degree, not an ordinary paper support having a high humid absorbability, from the view point of that not only the paper through image visibility just after recording by the ink-jet is sufficiently inhibited, but also the paper through image visibility is difficultly appeared since the degree of penetration of dye into the support is small. The use of the non-humid-absorbable support is further preferable since the paper through image visibility is further inhibited by inhibition of the penetration of dye into the support.
It is further found regarding the ink-jet recording sheet that a high quality print inhibited in the formation of stripe-like unevenness of image can be obtained when an image is recorded on one surface of the recording sheet after recording another image on the other surface of the paper. In such the case, the stripe-like unevenness of image is inhibited not only in the previously recorded image but also in the image later recorded on the other side of the recording sheet. Although the reason of such the effect is not clear yet, it is supposed as follows.
An ordinary ink-jet recording sheet using a paper support having a high humid absorbability tends to be cockled when an image is printed on one side thereof since the support absorbs the ink. In an ink-jet recording sheet using a support having a specified humid absorbability which difficultly absorbs humid, the cockling caused by printing an image on one side of the paper is inhibited. Accordingly, the formation of the stripe-like unevenness in the image printed on the back side of the paper is inhibited since the distance between the recording sheet and the printer head is constantly maintained so that the image can be uniformly printed.
From the above-mentioned viewpoint, it is further preferable that the support is a support having a Kobb-water-permeability of not more than 5 g/m2. The most preferable recording sheet using is one using a support of which Kobb-water-permeability of 0 g/m2.
Moreover, the recording sheet using the non-humid absorbable support is preferred since the formation of stripe-like unevenness of image can be inhibited and no cockling is occurred after printing on the both sides of the recording sheet. As a result of that, a printed image having a high quality feeling can be obtained.
A plastic. film support or a paper support coated with a plastic resin on the both sides thereof is preferable as the hydrophobic support, and a paper support comprising paper coated with a hydrophobic resin on the both sides thereof is particularly preferred.
The preferably usable support comprised of paper coated with the hydrophobic resin on the both sides thereof is described below.
The weight of the paper is preferably from 100 to 250 g, particularly from 120 to 220 g, per square meter as described above.
The weight of not less than 100 g/m2 is preferable, since a sufficient opacity can be obtained when the concentration of white pigment in the hydrophobic resin layer or the thickness of the hydrophobic resin layer is relatively small.
The weight of not more than 250 g/m2 is preferable since the stiffness of the support is suitable for transporting in the printer.
Paper, which is employed as a support in the present invention, is comprised of wood pulp as the main raw material, and if desired, in addition to the wood pulp, employed is synthetic pulp such as polypropylene and the like, as well as synthetic fiber such as nylon, polyester and the like. Employed as wood pulp may be, for example, any of LBKP, LBSP, NBKP, NBSP, LDP, NDP, LUKP, and NUKP.
Preferably employed as the aforementioned pulp is chemical pulp (sulfate pulp and sulfite pulp) which comprise minimal impurities. Further, pulp, which is subjected to bleaching to enhance whiteness, is also advantageous.
Into paper may suitably be incorporated sizing agents such as higher fatty acids, alkylketene dimers, and the like, white pigments such as calcium carbonate, talc, titanium oxide, and the like, paper strength enhancing agents such as starch, polyacrylamide, polyvinyl alcohol, and the like, fluorescent whitening agents, moisture maintaining agents such as polyethylene glycol, and the like, dispersing agents, softening agents such as tertiary ammonium compound, and the like.
During the paper making stage or after paper making, the paper may be further calendered and smoothened. Paper. density is commonly between 0.7 and 1.2 g/m2 (JIS P 8118). Further, the paper stiffness is preferably between 20 and 400 g under conditions specified in JIS P 8143.
Surface sizing agents may be applied onto the surface of the paper. Employed as surface-sizing agents may be the same as those described above, which can be incorporated into the base paper.
The pH of said paper, when measured employing a hot water extraction method specified in JIS P 8113, is preferably between 4 and 9.
Polyethylene, which is employed to laminate both surfaces of paper, is mainly low density polyethylene (LDPE) and/or high density polyethylene (HDPE). And further, other LDPE, polypropylene, and the like may be employed.
As for the titanium oxide, white pigment, barium sulfite, zinc oxide, calcium carbonate etc. are listed. Titanium oxide is particularly preferable among these. Specifically, rutile or anatase type titanium oxide is preferable. The amount of titanium oxide is commonly between 1 and 50 percent by weight with respect to polyolefin, and is preferably between 2 and 30 percent by weight.
The white pigment can be applied to one side or both sides of the polyolefin resin layers whereby translucency and whiteness is improved.
In order to adjust background whiteness, high heat resistant pigments and fluorescent whitening agents may be incorporated into the polyolefin layer.
Listed as color pigments may be, for example, ultramarine blue, iron blue, cobalt blue, phthalocyanine blue, manganese blue, serurian blue, tungsten blue, molybdenum blue, anthraquinone blue, and the like.
Cited as fluorescent whitening agents are, for example, dialkylaminocoumarin, bisdimethylaminostilbene, bismethylaminostilbene, 4-alkoxy-1,8-naphthalenedicarboxylic acid-N-alkylimide, bisbenzoxazolylethylene, dialkylstilbene, and the like.
Further, the aforementioned polyethylene laminated paper support preferably has the following properties.
Tensile strength: preferably 2 to 30 kg in the longitudinal direction, and 1 to 20 kg in the lateral direction, employing a method, specified in JIS P 8113.
Tearing strength: preferably 10 to 20 g in the longitudinal direction and 20 to 200 g in the lateral direction, employing a method, specified in JIS P 8116.
Whiteness: preferably L*=80 to 95, a*=xe2x88x923 to +5, and b*=xe2x88x926 to +2, wherein L*, a*, and b* are specified in JIS Z 8729.
Clark stiffness: the preferred support has a Clark stiffness of the recording sheet in the conveyance direction of 50 to 300 cm2/100.
The surface of the support may be highly smooth or fine grained. A glossy recording sheet is obtained when the support has the surface with a high smoothness. A fine grain surface recording sheet is obtained when the support has the fine grained surface.
When the polyolefin resin layer is provided on the both sides of the paper, the thickness and the composition of the resin layer and the surface quality of the each side may be the same or different. It is preferable that the surface qualities of each of the surfaces are the same when the recording sheet is used for making a print album.
Besides, when the recording sheet is used as a post card, it is not necessary that the qualities of the surfaces are the same and various surface qualities such as glossy, fine grain and matted surface may be optionally used in combination.
It is preferable for obtaining a glossy surface that the glossiness of the polyolefin resin layer is from 20 to 90% in the mirror surface glossiness at 60xc2x0 according to JIS Z 8741.
When the support is used for fine grain surface ink-jet recording sheet, the surface of the hydrophobic resin layer preferably has a center line average roughness measured by a standard length of 2.0 mm and a cut-off value of 0.8 mm according to JIS B 0601 of from 0.8 to 4.0 xcexcm and a mirror surface glossiness at 60xc2x0 according to JIS Z 8741 of from 10 to 40%. The mirror glossiness at 60xc2x0 of the surface of the hydrophobic resin for obtaining a matted surface is usually from 2 to 10%.
The glossy or fine grained surface of the polyolefin resin layer is formed by pressing the surface of the resin layer after laminating to a glossy roller or an embossing roller to form a fine pattern on the surface.
The embossing can be performed by two methods. In one of them, the embossing calendering is applied at a temperature near an ordinary temperature. In another method, the pattern is made by using a cooling roll on which the pattern is engraved while cooling the extruded molten resin layer just after the extrusion of the resin. The later method is preferable since the embossing can be accurately and uniformly performed with a relatively small pressure.
The ink absorption layer provided on the both sides of the support is described below. The ink absorption layers provided on the both sides of the support each contain a cationic substance.
When prints, on the both sides of each of which image are recorded by ink-jet recording, are piled up and left, the diffusion of the image is accelerated since the amount of remained solvent is raised in the piled portion where the images are formed on both sides of the print. Therefore, it is necessary to make higher the resistivity against the diffusion of the image dye compared to the case in which the image is printed on one side of the recording sheet.
Examples of the cationic substance usable in the invention include a cationic polymer, a coupling agent having a quaternary ammonium group, and an inorganic pigment having a cationic surface. Among them, the cationic polymer is particularly preferable.
Any known cationic polymer can be optionally selected from ones usually used in ink-jet recording sheet, and a cationic polymer having a tertiary amino group or a quaternary ammonium group is preferred. Examples of such the substance include polyethyleneimine, polyacrylamine, dicyandiamido-polyalkyleneamine, a condensation product of dialkylamine and epichlorohydrine, polyvinylamine, polyvinylpyridine, polyvinylimidazole, a condensation product of diaryldimethyl-ammonium salt, and a quaternarized compound of polyacrylic ester. The cationic polymer having a quaternary ammonium salt group is most preferable. The polymer having a quaternary ammonium salt group is a homopolymer of a monomer having a quaternary ammonium salt group or a copolymer of such the monomer and another copolymerizable monomer, and one having a weight average molecular weight of from 2,000 to 100,000, particularly from 3,000 to 80,000, is preferable.
The silane coupling agent having a quaternary ammonium salt group may be used by various procedures, for example, the coupling agent is added in a form of previously reacted with an inorganic fine particle, the coupling agent is added at an optional step from the coating to the drying of the ink absorption layer, and the coupling agent is over coated in a form of solution onto the previously formed ink absorption layer. Penetration of dyes to the opposite side of the paper can be inhibited by preventing bleeding in the ink receiving layer or diffusion to side closer to the support of the dyes by employing these cationic compounds. Content per 1 m2 of the compound to be employed is 1 to 50 g, preferably 2 to 30 g in each side for an inorganic pigment having cationic surface, and 0.1 to 10 g, preferably 0.2 to 5 g in each side for a cationic polymer or a silane coupling agent having quaternary ammonium base.
Examples of the inorganic fine particle having the cationic surface include alumina, pseudobaemite, and cation-modified colloidal silica.
The ink absorption layer is classified into an ink absorption layer mainly comprised of hydrophilic polymer and a porous ink absorption layer containing-inorganic fine particles in a high ratio to a hydrophilic binder.
The former is one which temporarily absorbs ink by swelling of the hydrophilic binder, and the image is remained, after evaporation of the solvent, in a form of dye imagewise contained in the binder in the original state.
The later is one in which the ink is absorbed and temporarily held in the porous layer. The later is preferable since it has a high ink absorbing ability. As a result of that, a blotch-like unevenness in the image is difficultly formed and the surface of it is dried state after the ink-jet recording. Accordingly, the later is preferable from the viewpoint of easy handling.
With respect to the ink-jet recording sheet for recording images on both sides thereof, it is necessary to sufficiently dry the paper after recording so that two or more sheets of the recording sheet can be piled without adhesion with each other when the ink absorption layer is swellable type. It is not so, the sheets each having the printed images on the both sides are easily adhered with each other.
The ink-absorptive layer is a void type ink absorbing layer comprised of a hydrophilic binder and fine inorganic particles. Listed as examples of fine inorganic particles can be white inorganic pigments such as soft calcium carbonate, heavy calcium carbonate, magnesium carbonate, kaolin, clay, talc, calcium sulfate, barium sulfate, titanium dioxide, zinc oxide, zinc hydroxide, zinc sulfide, zinc carbonate, hydrotalcite, aluminum silicate, diatomaceous earth, calcium silicate, magnesium silicate, synthesized amorphous silica, colloidal silica, alumina, colloidal alumina, pseudoboehmite, aluminum hydroxide, lithopone, zeolite, magnesium hydroxide, etc.
Primary particles without any treatment may be employed as such inorganic particles. Alternatively, secondary aggregated particles may be formed and employed.
In order to obtain high maximum density and excellent surface characteristics, fine particles are preferably employed so as to form an average particle size of less than 500 nm, more preferably less than 200 nm.
It is preferred to form fine voids in view of obtaining high density when the image is recorded by ink jet. For this purpose silica or pseudoboehmite is preferable. Particularly preferable is silica synthesized by gas phase method, colloidal silica and pseudoboehmite, having average particle diameter of 200 nm.
The average particle diameter of said fine inorganic particles is obtained as follows. Particles themselves, or the cross-section or the surface of a void layer is observed employing an electron microscope, and diameters of 100 randomly sampled particles are recorded. Then the average diameter is obtained as a simple average (a number average). The particle diameter of each particle as described herein is the diameter of an assumed circle which has the same area as the projection area of the particle.
Employed as hydrophilic polymers employed in the void layer are those which are employed in the swelling type ink absorption layer. However, the preferred hydrophilic polymer is polyvinyl alcohol.
Polyvinyl alcohols, which are preferably employed in the present invention, include, other than common polyvinyl alcohol obtained by hydrolyzing polyvinyl acetate, modified polyvinyl alcohols such as polyvinyl alcohol in which the terminals are subjected to cationic modification, modified polyvinyl alcohol comprising an anionic group, and the like.
Polyvinyl alcohol having an average degree of polymerization of at least 300, which is obtained by hydrolyzing vinyl acetate, is preferably employed, and particularly one having a degree of polymerization of 1,000 to 5,000 are preferably employed.
Polyvinyl alcohol having a saponification ratio of 70 to 100 percent is preferred and one having a saponification ratio of 80 to 99.5 percent is particularly preferred.
Ratio of inorganic fine particles to hydrophilic binder employed in the ink absorption layer is around 2:1 to 10:1, and particularly 3:1 to 8:1 is preferable.
Further, when the aforementioned void layer comprises polyvinyl alcohol as the hydrophilic polymer, it is preferred to employ a hardening agent to improve the film forming properties of the film as well as to increase the film strength.
In case that the hydrophilic binder is polyvinyl alcohol, that is most preferable example, epoxy compound, boric acid or its derivative is preferably incorporated as the hardening agent. The preferable examples are boric acid or its derivative.
Boric acids or salts thereof are oxygen acids and salts thereof having a boron atom as the central atom, and specifically include ortho-boric acid, meta-boric acid, hypoboric acid, tetraboric acid, pentaboric acid, and salts thereof.
The employed amount of boric acids or salts thereof may vary widely depending on the amount of fine inorganic particles and hydrophilic polymers in the coating composition. However, said amount is commonly between 1 and 60 percent by weight of the hydrophilic polymers, and is preferably between 5 and 40 percent by weight.
In the ink-receiving layer and an optional layer provided on demand of the ink-jet recording sheet, various additives can if desired be added.
For example, it is possible to add various additives, known in the art, such as UV absorbers described in Japanese Patent Publication Open to Public Inspection Nos. 57-74193, 57-87988 and 2-261476, anti-fading agents described in Japanese Patent Publication Open to Public Inspection Nos. 57-74192, 57-87989, 60-72785, 61-146591, 1-95091, 3-13376, etc., various anion, cation or nonion surface active agents, fluorescent whitening agents described in Japanese Patent Publication Open to Public Inspection Nos. 59-42993, 59-52689, 62-280069, 61-242871, 4-219266, etc., pH-adjusting agents such as sulfuric acid, phosphoric acid, acetic acid, citric acid, sodium hydroxide, potassium hydroxide, potassium carbonate, etc., antifoaming agents, lubricants such as diethylene glycol, etc., antiseptics, thickeners, antistatic agents, matting agents, etc.
The dry thickness of the porous ink absorption layer which is decided depending on the relation between the ink absorption amount, is usually from 20 to 70 xcexcm, and preferably from 30 to 50 xcexcm from the view point of necessity of that all the solvent of ink should be temporarily held in the layer. Particularly preferable dry thickness is from 35 to 50 xcexcm.
The ink absorption layer provided on the both sides of the support may be each comprised by a single layer or two or more sub-layers. When the layer is constituted by plural sub-layers, the compositions of each of such the sublayers may be different, and the sub-layers each preferably contains the cationic substance.
The composition, the thickness or the surface quality of each of the layers to be provided on the front side and the back side of the support may be the same or different from each other.
When the a colored image is principally printed on one side and a character image is principally printed on the other side such as in a card and post card, it is preferable to respectively design each of the ink absorption layers so that the properties of the each layer are suitable for the printing condition for each of the image.
On the other hand, when the recording sheet is used for making an album, the surface quality of both of the sides of the recording sheet are preferably the same, and the glossiness and the whiteness of the both sides may be different from each other. However, the ink absorbing properties of the both sides are preferably the same on both of the sides. Although glossiness and the whiteness of the front side and the back side may be different from each other, the ink absorption properties of the both sides are preferably almost the same.
When the ink absorption property of the front side and that of the back side of the recording sheet is different from each other, prints each quite different in the tone are obtained even when the same images are printed by the ink-jet printing under the same conditions. For inhibit such the problem as small as possible, it is preferable to make the ink-jet suitability of the both sides to as the same as possible. Accordingly, it is preferable that the ratio of D1/D2 is from 0.90 to 1.1, wherein the diameter of dot formed by adhering one droplet on the ink absorption layer of the front side and that of the back side are each D1 and D2, respectively.
The dot diameter is an average dot diameter of arbitrarily measured 20 dots. A single dot formed by shooting on a recording paper is photographed by a microscope or a CCD camera, and area of the dot is measured. Diameter of circle having same area of the dot is denoted as the dot diameter.
The spreading area of the ink droplet is within the range of almost the same when the ratio of D1/D2 is from 0.90 to 1.1, and the color reproducibility of the same image printed on the both sides of the recording sheet are preferably become to almost the same.
To make the ratio of D1/D2 to a value with in the above-mentioned range, the constitution of the ink absorption layer on the front side and that on the back side are made to substantially the same. In the ink absorption layers having substantially the same constitution, the same inorganic fine particle, the same hydrophilic binder and the same cationic substance are contained. It is preferable that the layer constitution is also made as similar as possible.
However, a tinting agent for giving a small difference of tone, a fluorescent whitening agent, or a matting agent may be different since they do not influence on the ink absorbing ability.
The ink absorbing capacity of the front side and the back side ma be different from each other within the range in which the image is not influenced.
The ratio of the ink absorbing capacity of the front side and that of the back side is usually from 0.8 to 1.2.
The static friction coefficient of the recording sheet according to the invention is preferably not more than 0.8 from the viewpoint of continuous transportation.
When the static friction coefficient is not more than 0.8, double feeding or stopping of paper supply is, difficultly occurred. It is particularly preferable to make to 0.8 or less. The static friction coefficient is usually not less than 0.2 although there is no limitation on the lower limit.
The static friction coefficient of the both sides of not more than 0.8 can be attained by controlling the surface properties, such as the surface quality and the slipping property, of the ink absorption layers of the both sides.
In concrete, the static friction coefficient can be easily controlled by an addition of matting agent for making a projection on the surface of the ink absorption layer on the both sides, or an addition of a slipping agent, such as a wax, silicone compound and a fluorinated compound, for giving a slipping property to the surface of the ink absorption layer. The addition of the matting agent is particularly effective.
As the matting agent, an inorganic particle, such as a particle of silica, calcium carbonate or talc, and an organic latex, such as a latex of polystyrene, polyethylene, or poly(methyl methacrylate). Among them the organic latex is preferred.
The matting agent having an average diameter of not less than 5 xcexcm is preferable since the slipping property can be improved by such the matting agent with a relatively small influence on the glossiness. The matting agent having an average diameter of from 7 to 30 xcexcm is particularly preferred.
The matting agent having a narrow diameter distribution is preferable since the loss of glossiness is small. The distribution degree, the ratio of the standard deviation of the size distribution of the particles to the average diameter of the particles, is preferably not more than 2, more preferably not more than 1.5.
The adding amount of the matting agent is usually from 0.01 to 0.5 g per square meter even though the amount may be changed depending on the diameter of the matting agent and the thickness of the ink absorption layer in which the matting agent is to be added.
The latex is a latex of polymer of a polymerizable monomer having at least one unsaturated bond in the molecular thereof or a copolymer of two or more kinds of the polymerizable monomer, which is synthesized by an emulsion polymerization method.